The invention relates generally to a low-hysteresis coupling for a shaft
There are many applications which require a coupling to couple a device to a shaft. Often, such couplings allow rotational movement of a shaft to be transferred to a device while not allowing translational movement of the shaft to be transferred to the device. In certain application, the coupling must substantially eliminate hysteresis (which is the lagging of a physical effect on a body behind its cause) in rotational movement of the shaft with respect to the device.
One application of the coupling is a torque sensor device which determines a torque input on a shaft which comprises a torsion bar longitudinally connected to a primary bar. The primary bar is relatively inflexible to a torque input, while the torsion bar torsionally flexes to a torque input. The magnitude of the torque input may be determined by measuring the rotation of the torsion bar relative to the primary bar. One of the difficulties in coupling the torsion bar to the primary bar is that the torsion and primary bar may not be coaxial due to manufacturing tolerances or design requirements. External forces acting perpendicularly to the longitudinal axis of the torsion bar may also cause translation of the torsion bar in an x-y direction of an x-y plane (which is perpendicular to the longitudinal axis of the torsion and primary bar) such that the torsion bar may become temporarily non-coaxial with the primary bar. In addition to connecting with non-coaxial bars, the coupling should transmit the relative rotation of the torsion bar to the torque sensor device without backlash. In other words, the coupling allows rotational movement of the shaft to be transferred to a device while not allowing translational movement of the shaft to be transferred to the device. Thus, the coupling substantially eliminates hysteresis in rotational movement of the primary bar with respect to the torque sensor device.
The torque sensor device may be used to accurately measure the input torque acting on a steering column shaft in an electronic power steering system of an automobile or truck. In this application, an input torque acts on the steering column shaft when an operator turns the steering wheel. The steering column shaft includes the primary bar and the torsion bar. The rotation of the torsion bar relative to the primary bar may be measured with a potentiometer. The torque sensor device may include a coupling which couples the torsion bar to the primary bar so that a sensor brush may slidingly contact a variable resistor as the torsion bar rotates relative to the primary bar. In order to accurately determine the relative rotation of the torsion bar, the coupling should accurately transfer the relative rotation of the torsion bar to the sensor brush with substantially no hysteresis and still allow the translation of the torsion bar in the x-y plane.
Several devices are currently available which couple non-coaxial shafts. However, none of the devices thus far appear to be without problems. One attempt to satisfy the needs discussed above is disclosed in U.S. Pat. No. 3,834,182 (Trask et al.). Referring to FIGS. 1 and 2, this patent describes a flexible coupler 20 for connecting nominally coaxial shafts drivingly connected to one another. The coupler 20 permits a limited amount of axial misalignment between the shafts. The coupler 20 comprises three basic elements: an enlarged cylindrical hub 22 fixed to a first shaft, a second smaller cylindrical flange 26 fixed to another shaft 28 in juxtaposition to the hub 22, and a xe2x80x9cfloatingxe2x80x9d annular ring 30 also juxtaposed with the hub 22 about the flange 26. Loose fitting complementary axial lugs 32 and notches 34 interconnect the hub 22 and ring 30, and loose fitting complementary radial lugs 36 and notches 38 are interfitted between the ring 30 and flange 26. The flange 26 and ring 30 are located relative to the hub 22 for axial clearance, permitting limited angular misalignment between the two shafts 24, 28. The flange 26 and circumjacent ring 30 form a planar surface juxtaposed with the inner planar surface 40 of the hub 22. However, due to the loose fitting complementary axial lugs 32 and notches 38 and the loose fitting complementary radial lugs 36 and notches 38, gaps 42 between the lugs 32 and notches 34 may lead to rotational play between the first and second shaft 24, 28.
U.S. Pat. No. 2,956,187 (Wood), U.S. Pat. No. 3,859,821 (Wallace), U.S. Pat. No. 4,357,137 (Brown), and U.S. Pat. No. 4,464,141 (Brown) appear to provide a coupling with less rotational play between a first and second shaft than the Trask patent. These patents describe a flexible coupling for transmitting power from a drive shaft to a driven shaft. The coupling includes a primary coupling member having a hub section for receiving and rotating with a first shaft, a flange section having a resilient insert therein, and a secondary coupling member located centrally within the resilient insert for receiving and rotating with a second shaft. The resilient insert is interference fitted into the primary coupling member, and the secondary coupling is interference fitted into the central region of the resilient insert The resilient insert is adequately flexible to allow for axial misalignments between the shafts. However, a slight rotational play appears to exist between the first and second shafts because the resilient insert flexes to an input torque acting on the shafts.
Another coupling with reduced rotational play is disclosed in U.S. Pat. No. 3,728,871 (Clijsen) which describes a coupling for connecting two approximately registering shafts. Referring to FIGS. 3 and 4, the coupling 50 comprises two connecting pieces 52, 54 respectively connected to a first 56 and second shaft 58. A loose coupling disc 60 is fitted between the two connecting pieces 52, 54 and couples the rotary movements of both connecting pieces 52, 54 to each other and has a limited play in two mutually perpendicular radial directions with respect to the individual connecting pieces 52, 54. Play in the direction of rotation is reduced by a resilient C-shaped spring member 62. One drawback of this coupling 50 appears to be that it is relatively complicated. This may result in an increase in manufacturing time and cost due to the numerous precision shaped components required. It also may result in a less reliable device because the inclusion of more components may translate into a statistically less reliable device.
Another coupling with reduced rotational play is a conventional Oldham coupling.
Referring to FIG. 5, the Oldham coupling 100 comprises three basic elements: a first member 102 connected to a first shaft at one end and having an axially extending tongue 104 at the other end, a second member 106 connected to a second shaft at one end and an axially extending tongue 108 at the other end, and a third member 110 positioned between the first member 102 and the second member 106. The third member 110 has a groove 112 at each end which slidingly mates with the respective tongues 104, 108. One drawback of the Oldham coupling 100 is that it appears to be relatively complicated. For the same reasons discussed above in regards to the Clijsen patent, the Oldham coupling may not satisfy certain needs for the torque sensor device.
Thus, there remains a need for a coupling that allows rotational movement of a shaft to be transferred to a device while not allowing translational movement of the shaft to be transferred to the device in an inexpensive, reliable, and rugged manner.
In accordance with the present invention, a coupling is coupled to a device in a manner that allows rotational movement of a shaft to be transferred to a device while not allowing translational movement of the shaft to be transferred to the device. The coupling is particularly suited for any device which requires substantially no hysteresis in rotational movement of the shaft with respect to the device. The present invention achieves the objective of coupling a shaft to a device in an inexpensive, reliable, and rugged manner.
The coupling of the present invention is particularly useful in an angular-position and torque sensor assembly for an electronic power assisted rack and pinion steering system of an automobile or truck. The steering system includes a steering wheel, column shaft, sensor assembly, steering gear, servo motor, pinion, and rack. The steering wheel is coupled to one end of the column shaft, and the opposite end of the column shaft is coupled to the steering gear. The other end of the steering gear is connected to a pinion which is rotatively coupled to the rack such that an operator turning the steering wheel causes the pinion to rotate along the rack. The rack moves longitudinally and turns the tires of the automobile. The servo motor is connected to the steering gear to provide power assist. The sensor assembly is coupled to the column shaft and accurately determines the angular position of the column shaft and input torque acting on the column shaft by the operator turning the steering wheel. Based on the data received from the sensor assembly, the controller processes the data and directs the rotational direction and power output of the servo motor such that a larger input torque results in providing more power to the servo motor. Thus, the steering system provides an appropriate level of power assistance to aid in steering.
Generally, in accordance with an exemplary illustrative embodiment of the present invention, the sensor assembly may comprise (1) a position substrate having a slip ring and a variable resistor ring, (2) a first rotating member rotating about the position substrate and having an electrical contact on the bottom side and a second variable resistor on the top side, and (3) a second rotating member rotating about the first rotating member and having a coupling on the top side and an electrical contact on the bottom side such that the electrical contact of the second rotating member slidingly contacts the second variable resistor.
The coupling comprises an inner member, an outer member, and a base member disposed outside the outer member. First rails connect the inner member to the outer member, and the first rails are aligned substantially perpendicular to a reference axis (the reference axis is on an x-y plane substantially parallel to the top surfaces of the inner member, outer member, and base member) to allow the inner member to be readily displaced relative to the outer member only in a direction substantially parallel to the reference axis. Second rails connect the outer member to the base member, and the second rails are aligned substantially parallel to the reference axis to allow the outer member to be readily displaced relative to the base member only in a direction substantially perpendicular to the axis. Due to the configuration of the first and second rails, the inner member is free to move relative to the base member in an x-y direction of the x-y-plane while being rotatively fixed in a z axis which is perpendicular to the x-y plane.
Other objects, features, and advantages of the present invention will become apparent from a consideration of the following detailed description